1. Field of the Invention
This invention relates generally to sonar apparatus and more particularly to a sonar system which adaptively modifies its vertical beamwidth and steering angle in accordance with the water depth in which it is operating so as to maximize the range at which detection and classification of underwater targets is possible.
2. Description of Related Art
The shift in emphasis from blue water to the littoral naval operations has increased the importance of capably detecting targets, such as mines, in shallow waters. However, detecting and classifying these targets, particularly mines in shallow water, is complicated by the proximity of the boundaries within which the target threat is found. In the case of Very Shallow Water (VSW), ranging from 10 feet to 40 feet water depth, the extreme proximity of the sea surface and bottom result in highly reverberant, multi-path environment with severe impact on the operation of conventional mine-hunting sonars, such as side looking sonars (SLS).
As sonar range potential increases due to technical advances such as Synthetic Aperture Sonar (SAS), the effect of boundary reverberation and multi-path is extended to deeper waters in the same way that the VSW affects state of the art sonar systems, i.e. as range is extended, deeper waters will tend to exhibit the same limitations as "shallow" water. Although particularly directed to VSW, the technique which will be disclosed hereinafter is also applicable to water depths that are substantially in excess of those found in the VSW environment. To regain a substantial measure of performance enjoyed in deeper water and to increase the search rate to acceptable levels, similar arrays must be modified to operate in this environment.
The basic problem associated with the VSW environment, or in water depths that are shallow with respect to sonar range, is depicted in FIG. 1. As shown in FIG. 1, a side looking sonar (SLS) 10 is operating against a mine 12 residing on the bottom 14 of a body of water having a surface indicated by reference numeral 16. Both the return sonar signal 18 and the acoustic shadow 20 behind it, where the mine body blocks the acoustic energy from hitting the bottom 14, compete with multi-path 22, and surface reverberation 24 arriving at the same time. Because both detection and classification rely on the target highlight and shadow so as to indicate both mine presence and dimension, operation is compromised.
Two possible methods to combat the adverse effects of the VSW environment include: (a) narrowing the vertical beamwidth to decrease the vulnerability to both multi-path and direct verberation; and (b) the placement of a dynamic null in the vertical beam pattern to counteract direct surface verberation. In the latter case, both the geometry of the situation and the sound velocity has to be known to enable the null to track the expected source point of the surface reverberation. In the multi-path environment, however, a null directed at the surface will not remove all the interfering paths. Accordingly, the approach chosen in the subject invention is to dynamically narrow the vertical beamwidth until sufficient gain is achieved against interference.